1. Field of the Invention
The present invention relates to a wireless communication system and a frequency hopping method therefor, and a base station and a mobile station, and more particularly, to a wireless communication system to which frequency hopping is applied and a frequency hopping method therefor, and a base station and a mobile station.
2. Description of the Related Art
FIG. 1 is a timing chart exemplifying frequency hopping used in a related wireless communication system. FIG. 1 illustrates as an example a case where a transmit carrier frequency is changed every one time slot (one slot) on the transmitting side.
As illustrated in FIG. 1, each of time slots 100 (100-1 to 100-3) includes a reference signal 101 and data and control signals 102. The reference signal 101 represents a so-called “pilot signal”.
Referring to FIG. 1, illustrated is a case where the transmit carrier frequency is f1 during a time slot 100-1, the transmit carrier frequency falls to f3 during a subsequent time slot 100-2, and the transmit carrier frequency rises to f2 during a further subsequent time slot 100-3 (where f1>f2>f3).
In this manner, in the wireless communication system to which the frequency hopping method is applied, the transmit carrier frequency is shifted up and down in accordance with a predetermined pattern.
In such a wireless communication system as described above, by correcting a difference between a transmit carrier frequency from a mobile station and a received carrier frequency on a base station side, an uplink (direction from mobile station to base station) receive function on the base station side can be improved.
For example, when a frequency drift between the mobile station and the base station is 1 kHz and the length of a time slot is 0.5 ms, a baseband received signal appears to have rotated by 180° (i.e. 0.5 ms×1 kHz×360°=180°) during one time slot. As a result, the frequency drift causes deterioration of receiving characteristics in a case where a signal modulated by binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), or the like is subjected to a phase judgment.
As one exemplary method of correcting the frequency drift in a receiver, there is known an automatic frequency control (AFC). With a common method for the AFC, the base station observes a phase shift amount of a received signal that is regularly transmitted from the mobile station and corresponds to a known pilot signal, and rotates in antiphase the received signal by the observed phase shift amount, thereby correcting the frequency drift of the received signal.
Meanwhile, Japanese Laid-Open Patent Publication No. 2004-069297 A (Patent Document 1) discloses an example of a technique in which a measurement period is switched over, while Japanese Laid-Open Patent Publication No. 07-321710 A (Patent Document 2) discloses an example of another technique in which a period of hopping frequency switching is changed.
The technique disclosed in Patent Document 1 relates to an apparatus for measuring a gas flow rate, in which a measurement period of a flow rate is switched over between a continuous period and an intermittent period according to the size of the initial flow rate. To give an example, when the flow rate is large and a measurement period immediately before is the intermittent period, the measurement period is switched to the continuous period.
The other technique disclosed in Patent Document 2 relates to higher speed synchronization acquisition for a communication apparatus that uses a frequency hopping method. In the technique, switching periods of hopping frequency allocated to the first three codes from the head of each spread code series that is output from a spread code generating circuit is set shorter than switching periods of hopping frequency allocated to the other codes.
However, regarding the wireless communication system in which frequency hopping is conducted on a time slot basis as illustrated in FIG. 1, the phase of a received signal on the base station side is changed according to the change of frequency. Therefore, the system has a problem in that a phase shift amount at the same frequency in a frequency hopping system cannot be measured.
Meanwhile, the techniques disclosed in Patent Documents 1 and 2 are techniques in which a measurement period and a hopping period are switched over, respectively, but any of those techniques is quite different in object and configuration from the present invention. Thus, those techniques are different from the present invention, which cannot achieve the same effect as the present invention.